Moro's power draining made him able to defeat Super Saiyan Blue Goku and Vegeta. But in the fight with Majin Buu there isnt any sign they are working at all. Why Moro's power draining dont work in Majin Buu?
Majin Buu used the Supreme Kaioshin's magic to defend himself against Moro's power drain.
As explained in Dragon Ball Super's chapters 48 - 49, a team of Galactic Patrol hypno-therapists managed to awaken the dormant consciousness of the Supreme Kaioshin within Good Buu. (As a reminder, Good Buu is called as such because the Supreme Kaioshin's appearance and personality influence him).
Majin Buu is now able to swap his consciousness with the Supreme Kaioshin's at any time. The Supreme Kaioshin already fought against Moro and developed several magical countermeasures, which notably protect him from Moro's power drain.